The present invention relates to an external magnetic field application apparatus of a magneto-optical disk apparatus which operates according to a laser power modulation system.
The magneto-optical disk apparatus using a laser power modulation system is provided with an external magnetic field application apparatus which operates in a recording mode such that it applies, when writing, a magnetic field in one direction to a position on a magneto-optical disk impinged by a light beam emitted from an optical head. For erasing purposes, it applies a magnetic field in a reverse direction.
One such external magnetic field application apparatus of a conventional magneto-optical disk apparatus is disclosed in the Japanese Laid-Open Patent Application Ser. No. 4-278241.
The conventional apparatus disclosed in the above application is constructed such that a pivotable support plate (arm) provided with a permanent magnet having magnetic poles of opposite polarities adjacent each other and with a driving permanent magnet, is assembled, by a pin member, to a cartridge holder into which a cartridge accommodating a magneto-optical disk is inserted. In this application yokes are provided with a driving coil facing the driving permanent magnet assembled to the cartridge holder.
A current is applied to a driving coil so that the arm is moved around the pin member between two extreme positions. As this movement of the arm is repeated, the magnetic field applied to a portion of the magneto-optical disk directly opposite the optical head is reversed accordingly.
However, the conventional apparatus described above has one drawback in that, since the driving permanent magnet is provided in the arm, the entire arm becomes relatively heavy. As a result, the moment of inertia of the pivotable support plate is great and the frequency at which the arm reverses its position is low. This causes magneto-optical disk writing and erasing operation to be slowed down, and requires that a relatively large current be supplied to the driving coil.
Also, the arrangement whereby the yokes provided with the driving coil is assembled to the cartridge holder invites an increase in the temperature of the surface of the magneto-optical disk. This brings about the possibility that information cannot be accurately written to and read from the magneto-optical disk. Further, in the above described construction in which the arm and the yokes are provided in the cartridge holder, a limit is imposed on the space in which the yokes etc. are mounted due to the requirement of making the magneto-optical disk apparatus thin. Hence, there is a problem that the degree of freedom in constructing the yokes etc. is small.
In the above described conventional technology, the arm provided with a permanent magnet having magnetic poles of opposite polarities adjacent each other is moved around the pin member between two extreme positions so that the magnetic field applied to a portion of the magneto-optical disk directly opposite the optical head is reversed according to the movement of the arm. In this construction, the size (width) of the permanent magnet, the length of the arm, the position of the pivot around which the arm is rotatable (the position of the pin member) have to be adjusted as accurately as possible so as to obtain a uniform magnetic field both when the portion of the magneto-optical disk directly opposite the optical head resides toward the center of the disk or when that portion resides toward the periphery thereof.
Moreover, the construction in which the means for moving the arm is formed of a driving coil, a driving permanent magnet and the like has a problem in that the driving coil and the like are relatively expensive, thus preventing reducing the cost of the entire apparatus.